Understanding detergent effects on lipid membranes: a model study of lysolipids

Abstract

Lysolipids and fatty acids are the natural products formed by the hydrolysis of phospholipids. Lysolipids and fatty acids form micelles in solution and acts as detergents in the presence of lipid membranes. In this study, we investigate the detergent strength of a homologous series of lyso-phosphatidylcholine lipids (LPCs) on 1-palmitoyl-2-oleyl-sn-glycerol-3-phosphatidylcholine (POPC) lipid membranes by use of isothermal titration calorimetry and vesicle fluctuation analysis. The membrane partition coefficient (K) and critical micelle concentration (cmc) are determined by isothermal titration calorimetry and found to obey an inverse proportionality relation (cmc.K approximately 0.05-0.3). The partition coefficient and critical micelle concentration are used for the analysis of the effect of LPCs on the membrane bending rigidity. The dependency of the bending rigidity on LPC membrane coverage has been analyzed in terms of a phenomenological model based on continuum elastic theory, which yields information about the curvature-inducing properties of the LPC molecule. The results reveal: 1), an increase in the partition coefficient with increasing LPC acyl-chain length; and 2), that the degree of acyl-chain mismatch between LPC and POPC determines the magnitude of the membrane mechanical perturbation per LPC molecule in the membrane. Finally, the three-stage model describing detergent membrane interaction has been extended by a parameter D(MCI), which governs the membrane curvature stability in the detergent concentration range below the cmc-value of the LPC molecule.

Figure 1

Isothermal titration calorimetry injecting POPC LUVs into LPC12 at 25°C. (a) Heat-spikes from 2 μL 65.4 mM POPC LUVs injected into a 200 μM LPC12 solution containing 75-mM glucose. (b) Peak integrals of the heat-spikes shown in panel a as a function of injection number. The peak integrals are fitted using Eqs. 3–5 yielding the partition coefficient, K, the molar enthalpy of partitioning, $\Delta H_{\mathrm{w}}^{\text{mem}}$, and the heat of dilution, q_dil, as fitting parameters.

Figure 2

Plot of the effective bending rigidity as a function of (a) the LPC bulk concentration given in units of the LPC cmc-value and (b) the LPC membrane molar fraction, x_p, given by $x_{\mathrm{p}}\simeq C_{\mathrm{p}}^{\text{LPC}}/C_{\text{Lip}}=K{C}_{\text{LPC}}$ for C_LPC < cmc. Lines representing the fit of Eq. 7 are shown in panel a and guidelines emphasizing the differences in LPC influence on κ_eff as a function of x_b are shown in panel b.

Figure 3

Illustration of the LPC equilibria in the presence of a lipid membrane. The partitioning of LPC is given by the partition coefficient, K, and the free energy of partitioning, $\Delta G_{\mathrm{w}}^{\text{mem}}$. The micelle formation equilibrium is described by the cmc-value and the free energy of transferring a detergent monomer from bulk to micelle, $\Delta G_{\mathrm{w}}^{\text{mic}}$. The incorporation of LPC molecules in the membrane bilayer is illustrated, showing increased degree of acyl-chain mismatch going from LPC16 to LPC12.